The present invention relates to multi-stage Gifford McMahon (GM) type pulse tube refrigerators as applied to recondensing helium in a MRI magnet. GM type refrigerators use compressors that supply gas at a nearly constant high pressure and receive gas at a nearly constant low pressure to an expander. The expander runs at a low speed relative to the compressor by virtue of a valve mechanism that alternately lets gas in and out of the expander. Gifford in U.S. Pat. No. 3,119,237 describes a version of a GM expander with a pneumatic drive. The GM cycle has proven to be the best means of producing a small amount of cooling below about 20 K because the expander can run at 1 to 2 Hz.
A Pulse Tube refrigerator was first described by Gifford in U.S. Pat. No. 3,237,421, which shows a pair of valves, like the earlier GM refrigerators, connected to the warm end of a regenerator, which in turn is connected at the cold end to a pulse tube. Early work with pulse tube refrigerators in the mid 1960s is described in a paper by R. C. Longsworth, ‘Early pulse tube refrigerator developments’, Cryocoolers 9, 1997, p. 261-268. Single-stage, two-stage, four stages with inter-phasing, and co-axial designs were studied. All had the warm ends of the pulse tube closed and all but the co-axial design had the pulse tubes separate from the regenerators. While cryogenic temperatures were achieved with these early pulse tubes the efficiency was not good enough to compete with GM type refrigerators. U.S. Pat. No. 4,606,201 by Longsworth describes a different type of pneumatic drive for a GM type expander that uses gas flowing through an orifice to and from a buffer volume to control the displacer.
A significant improvement was reported by E. I. Mikulin, A. A. Tarasow and M. P. Shkrebyonock, ‘Low temperature expansion (orifice type) pulse tube’, Advances in Cryogenic Engineering, Vol. 29, 1984, p. 629-637 in 1984, and a lot of interest ensued in looking for further improvements. This initial improvement used an orifice and a buffer volume connected to the warm end of the pulse tube to control the motion of the “gas piston” in the pulse tube to produce more cooling each cycle. In effect the gas piston replaced the solid piston, often referred to as a displacer, in U.S. Pat. No. 4,606,201. Subsequent work focused on both means to improve the control of the gas piston and on improving the configuration of the pulse tube expander. S. Zhu and P. Wu, ‘Double inlet pulse tube refrigerators: an important improvement’ Cryogenics, vol. 30, 1990, p. 514, describe a double orifice means of controlling the gas piston.
Gao, U.S. Pat. No. 6,256,998 describes a means of controlling the gas pistons in a two-stage pulse tube that works well at 4 K. Chan et al in U.S. Pat. No. 5,107,683 describe the extension of the second stage of a pulse tube from the second stage heat station to ambient temperature. This concept is one of several configurations studied by J. L. Gao and Y. Matsubara, ‘Experimental investigation of 4 K pulse tube refrigerator’, Cryogenics 1994 Vol. 34, p. 25 that has proven to work well for two-stage 4 K pulse tubes. The arrangements that were studied all had the pulse tubes separate from the regenerators.
A co-axial pulse tube with single orifice control was reported in 1986 by R. N. Richardson. ‘Pulse tube refrigerator-an alternative cryocooler?’ Cryogenics, 1986, 26(6): p. 331-340. Inoue et al in JP HO7-260269 describe a two-stage pulse tube in which the regenerators and pulse tubes are co-axial. The design has the second stage pulse tube in the center, extending from the second stage heat station to ambient temperature, surrounded by the first and second stage regenerators. The first stage pulse tube is a co-axial annular volume on the outside of the first stage regenerator. The central feature of this patent is the placement of heat exchangers within the pulse tubes to help equalize the temperature profiles in the pulse tubes with the temperature profiles in the regenerators. Temperature differences between the pulse tubes and the regenerators are not a problem when the tubes are separate from the regenerator and the pulse tube is surrounded by vacuum. The temperature differences however result in convective thermal losses when a conventional pulse tube is mounted in the helium atmosphere in the neck tube of a MRI cryostat.
Losses associated with temperature differences in co-axial pulse tubes were studied by L. W. Yang, J. T. Liang, Y. Zhou, and J. J. Wang, Research of two-stage co-axial pulse tube coolers driven by a valveless compressor, Cryocoolers 10, 1999, p. 233-238 and by K. Yuan, J. T. Liang, Y. L. Ju, Experimental investigation of a G-M type co-axial pulse tube cryocooler, Cryocoolers 12, 2001, p. 317-323. First they found it best to have the pulse tubes in the center surrounded by the regenerators in the annular space around the pulse tube. Losses were minimized by superimposing “dc” flow that brought warm gas down the pulse tubes over many cycles. When running in a vacuum they found that an external second stage pulse tube was more efficient than a co-axial second stage.
Mastrup et al., U.S. Pat. No. 5,613,365 describes a single stage concentric (co-axial) Stirling cycle pulse tube in which a central pulse tube has a thick wall made of low thermal conductivity material that provides a high degree of insulation from the annular regenerator on the outside. This idea was extended by Rattay et al., U.S. Pat. No. 5,680,768, in which the surrounding vacuum extends into a gap between the pulse tube wall and the inner wall of the regenerator.
Another means of insulating the wall of a pulse tube is described by Mitchell in U.S. Pat. No. 6,619,046. The advantages of the cold end heat exchanger in single stage co-axial pulse tubes are cited in Chrysler et al., U.S. Pat. No. 5,303,555, and by Kim et al., U.S. Pat. No. 6,484,515.
The problems associated with recondensing helium in a MRI magnet have been addressed by Longsworth in U.S. Pat. No. 4,606,201. A two-stage GM expander that has a minimum temperature of 10 K precools gas in a JT heat exchanger that produces cooling at 4 K. The JT heat exchanger is coiled around the GM expander so that the temperature of both the JT heat exchanger and the expander get progressively colder between the warm and cold ends. The expander assembly is mounted in the neck tube of a MRI magnet where it is surrounded by helium gas that is thermally stratified by virtue of being vertically oriented with the cold end down. The 4 K heat station has extended surface to recondense He. Refrigeration is transferred to cold shields in the MRI cryostat at two heat stations which are at temperatures of approximately 60 K and 15 K. Mating conical heat stations and bellows in the neck tube enable both heat stations to engage as the warm flange is bolted down and sealed with a face type “O” ring.
Longsworth, U.S. Pat. No. 4,484,458, had previously described the concentric GM/JT expander which had straight heat stations and a radial type “O” ring seal at the warm flange. This permits the expander to be moved axially to establish a desired position of the expander heat stations relative to the neck tube heat stations.
Advances in pulse tube technology and MRI cryostat design now make it possible to use a two stage pulse tube to cool a single shield at about 40 K and recondense helium at about 4 K. Two-stage pulse tube expanders are preferred over two-stage GM expanders because they have less vibration and thus generate less noise in the MRI signal. When a pulse tube of conventional design, with the pulse tubes parallel to the regenerators, is inserted into the neck tube of a MRI magnet it is found that helium gas in the neck tube circulates between the pulse tubes and the regenerators due to the temperature differences between them. This results in a serious loss of refrigeration.
Stautner et al., PCT patent application WO 03/036207 A2, explains the problem for a conventional two-stage 4 K pulse tube and offers a solution in the form of a sleeve that surrounds the pulse tube assembly and has insulation packed around the tubes. The sleeve has a heat station at about 40 K and a recondenser at the cold end and can be easily removed from the neck tube to be serviced.
Daniels et al., PCT patent application WO 03/036190 A1, offers another solution to the problem of convection losses of a conventional two-stage 4 K pulse tube in a MRI neck tube. Insulated sleeves around the pulse tubes and regenerators reduce convective losses when the pulse tube is mounted in the helium gas in a MRI neck tube.
One of the objects of this invention is to provide a design that reduces the vibration that is transmitted to an MRI cryostat by the expander.
It is an object of this invention to provide an easy way to remove the pulse tube expander for service.
It is an object of this invention to provide a co-axial design that is more compact than conventional parallel tube design.
It is an object of this invention to provide a method of eliminating convective losses due to heat transfer between the pulse tubes and regenerators.
It is a further object of this invention to provide a method for optimizing the design of a co-axial pulse tube.